1. Field of the Invention
This invention relates generally to a method and apparatus for electroplating and, more particularly, to a method and apparatus for holding a workpiece during electroplating with copper or other metals, for example, electroplating of semiconductor wafers.
2. Discussion of the Related Art
To meet the demands for higher and higher circuit speed in ultra large scale integrated (ULSI) circuit devices, it has become imperative to reduce the size of wiring structures in ULSI devices. For example, with respect to devices in any given dielectric film, as clock frequencies increase and as interconnect ground rules decrease, on-chip resistance-capacitance (RC) time delays increase. The restraints imposed by the increased RC time constants, becomes one of the critical impediments to achieving higher and higher circuit speeds. Moreover, the increasing interconnect current density accompanying the decreasing line and via dimensions is a source of reliability concern, especially in aluminum (Al) based metallurgy, the backbone of present day chip wiring. The incorporation of lower resistivity copper (Cu) and its microalloys in semiconductor chip wiring structures, results in improved chip performances and superior reliability when compared to Al based interconnect metallurgies.
Electrodeposition methods are rapidly emerging as a method of choice for chip metallization in high performance ULSI circuits. This is especially so when copper structures are used for chip wiring. The emergence of copper electrodeposition, as opposed to chemical vapor deposition (CVD) of copper, copper sputtering, and other copper deposition methods has resulted from an evaluation of various copper deposition methods for submicron and deep submicron chip wiring.
Electrodeposition, alternatively referred to as electroplating, is a process for depositing films of metal or alloy on a workpiece article such as various electronic components, for example. In electroplating, the article is placed in a suitable electrolyte bath containing ions of a metal to be deposited. The article forms a cathode which is connected to the negative terminal of a power supply, and a suitable anode is connected to the positive terminal of the power supply. Electrical current flows between the anode and cathode through the electrolyte, and metal is deposited on the article by an electrochemical reaction.
In interconnect metallization schemes, the uniformity of metal distribution on the substrate has been a source of concern. The better the uniformity, the more desirable the process. In electrodeposited film, to achieve a uniform metal deposit on the cathode, the design of the workpiece may require use of an anode or cathode shielding, or the use of auxiliary cathode (known in the art as a "thief" and hereinafter referred to as a thief). A thief is often incorporated around a plating workpiece, either to improve the uniformity of electrodeposited metal on the workpiece or to control the profile of the deposited metal. Generally, the workpiece is disposed in close proximity to the auxiliary electrode during a plating process. To prevent the thief from shorting to the workpiece, a thin insulating spacer, typically on the order of one millimeter (1 mm), is used to isolate the thief from the workpiece. The later arrangement tends to produce the desired metal uniformity on the workpiece or substrate of interest; however, during plating, an undesired buildup of plated metal over the insulating spacer between the thief and the substrate often occurs. Such is the case in the electrodeposition of copper for semiconductor chip wiring and/or interconnection applications. The buildup of plated metal over the thief/substrate insulating spacer undesirably shorts the thief to the workpiece. The bridging of the thief to the workpiece or wafer distorts the desired metal distribution profile on the workpiece or wafer, thus producing a defective part, and further requiring a rework operation.
To correct for the undesired bridging, very periodic inspections of the cathode or cathode assembly for workpiece/thief bridging and replacement of the thief/workpiece assembly are required. Such a periodic replacement of thief/workpiece cathode assembly results in losses due to equipment down time, and the cost/expense of frequent inspection of the thief/wafer isolator for conditions of undesired metal bridging.
It is, thus, desirable to provide a method and apparatus for overcoming the problems associated with the undesired premature bridging between a workpiece and auxiliary electrode during an electroplating process.